1. Field of the Invention
This invention relates to an electro-optical memory cell with an electric-field induced modification of an optical property, such as, refractive index. More particularly, this invention relates to an electro-optical memory cell with a non-volatile programmable refractive index.
2. Description of Related Art
Electro-optical devices such as switches, filters, tunable lasers, etc., that are characterized by the use of an electric-field induced modification of a refractive index are known, for example, from U.S. Pat. No. 3,883,220, from U.S. Pat. No. 4,787,691, and from U.S. Pat. No. 7,009,759. These optical elements include crystalline insulating materials such as LiNbO3, exhibiting a pyroelectric effect, semiconducting materials such as Si, exhibiting a plasma dispersion effect, or including a layered Li-Ion-accumulator-like structure where a Li ion migration can contribute to a change of refractive index.
A main drawback of these optical elements is that the electric power has to be applied permanently to keep the change in refractive index, that is, these electro-optical elements are “volatile”. A further disadvantage is that electro-optical elements including crystalline materials suffer from polarization dependency, birefringence, which requires additional structures to re-attain polarization-independent device operation.
Moreover, crystalline materials require a large thermal budget and therefore cannot be integrated through back-end-of-line (BEOL) fabrication. In addition, the above mentioned electro-optical elements including semiconducting materials or Li ions are only suited for the infrared wavelength region, as they can strongly absorb visible light.
An example of a non-volatile electro-optical switch is disclosed in U.S. Pat. No. 6,628,450. This example includes floating charge modulated regions to be charged across tunneling barriers. This example, however, relies on free electrons in semiconducting material and is therefore limited to infrared wavelengths and brings about comparably high absorption. Also, it has a complicated structure with several interfaces that can contribute to scattering. Finally, the voltage to be applied across two tunneling barriers taught by U.S. Pat. No. 6,628,450 is substantial.
Despite availability of variety of non-volatile electro-optical devices, such as those described in the commonly owned U.S. application Ser. No. 12/353,517 filed Jan. 14, 2009, there are few reports of electro-optical memory cells that operate by an electric-field induced modification of an optical property, such as, refractive index. However, there are no reports of electro-optical memory cells which have a non-volatile programmable refractive index.
Accordingly, it is desirable to provide a device that has a simple construction with few layer-to-layer interfaces and is well-suited for use in transmission of light in the infrared or visible range of the spectrum.